Many human genetic disorders are caused by mutations that impair protein folding and trafficking. Even though the mutated proteins may be produced in normal amounts and may even be functionally competent, problems can arise because the mutated proteins do not fold properly and/or are not processed and trafficked correctly. Consequently, such proteins do not reach their intended cellular location and tend to accumulate in the endoplasmic reticulum (ER) or other organdies where they are prone to aggregation. The relative importance of these contributions to cellular dysfunction and disease varies among diseases, and may even differ from patient to patient and potentially from cell type to cell type. There are some conditions where loss of protein function is the primary cause of disease, and others for which a toxic-gain-of function is caused by aggregation, and excessive ER retention is the primary source of pathology. In the case of α-1-antitrypsin deficiency, both loss-of-function and toxic-gain-of-function contribute to disease pathology.
Alpha-1-antitrypsin is a protein made in hepatocytes and secreted from the liver into the blood where it functions to limit neutrophil elastase activity in the lung. A deficiency in α-1-antitrypsin can lead to emphysema, as a result of increased degradation of lung connective tissue. In many patients, alpha-1-antitrypsin deficiency is caused by a E342K missense mutation therein, referred to as the Z mutant (Z-AT). The quality control mechanisms of the ER lead to retention and accumulation of Z-AT in hepatocytes, causing damage to the liver and reducing plasma levels of alpha-1-antitrypsin as a result of reduced secretion into the blood. Thus, lung disease associated with alpha-1-antitrypsin deficiency is caused by a loss-of-alpha-1-antitrypsin function, while liver disease occurs when the Z-AT accumulates to toxic levels in liver cells (toxic-gain-of-function). Since monomeric Z-AT retains the same specific activity as wild type α-1-antitrypsin (M-AT), treatment strategies that increase secretion of Z-AT by reducing its ER retention should protect against both liver and lung damage.
Studies have demonstrated that some compounds, such as 4-phenylbutyric acid, can increase secretion of Z-AT from cells. Additionally, some small peptides (e.g., Ac-TTAI-NH2) and citrate have been shown to block in vitro polymerization of Z-AT. However, the use of such osmolytes to promote protein folding is very limited as they require very high cellular concentrations and lack target specificity. Desirably, to effect protein folding and secretion in vivo, a compound must be able to penetrate the ER of liver cells, have a high affinity for Z-AT, and block polymerization thereof with minimal toxicity and side effects.
Current therapy for conditions associated with α-1-antitrypsin deficiency is limited to protein replacement therapy with M-AT derived from human plasma, typically dosed on a weekly basis. While such therapy is effective for lung pathology including emphysema, it has no effect on liver disease caused by the accumulation of polymerized Z-AT in the ER of hepatocytes. For the 10-15% of homozygotes for Z-AT afflicted with early-onset liver disease including cirrhosis and hepatocellular carcinoma, liver transplantation is the only treatment option available. Furthermore, accumulation of polymerized Z-AT in lung epithelium has a chemoattractant effect on neutrophils, which may cause further destruction of connective lung tissue. Thus, there is a need for therapeutics and methods that address disease conditions associated with both alpha-1-antitrypsin deficiency as well as toxic accumulation of alpha-1-antitrypsin.